Present status and future of DC photoemission electron guns for high power, high brightness applications

Transcription

1 Present status and future of DC photoemission electron guns for high power, high brightness applications DC photoemission electron guns using GaAs cathodes have been in use to produce polarized electrons for nuclear physics experiments for about 3 decades with great success, albeit at currents below 1 micro-ampere. In the late 1990's, this type of technology was adopted by the JLab FEL to generate un-polarized electron beam but at higher current, in the order of 10-mA, allowing the FEL to establish a new average power record. The electron beam quality, the pulse train versatility and the many thousands of coulombs delivered make this type of gun an ideal candidate for multiple applications such as high power FELs, X-ray FELs, electron-ion colliders, etc. A summary of the principle behind DC photoemission guns, the current status of the technology and ongoing developments worldwide towards a robust system will be presented. High Brightness High Power Workshop UCLA, January C. Hernandez-Garcia Jefferson Lab, Newport News, VA, USA

2 GaAs Photoemission Guns* An invention over 30 years old. Spin-dependent electron scattering tells us something about the structure of the target (e.g., molecule, atom, proton, neutron). Good tool for Condensed Matter and Atomic Physics research. Great tool for High Energy and Nuclear Physics research. Relatively new application; Un-polarized electrons for Light Sources, but already delivered > 15,000 Coulombs at ~10mA CW, in operation for ~10 years * Courtesy of Matt Poelker. CEBAF Polarized Source Group.

3 JLab FEL gun delivered over 7000 Coulombs and over 900 hours of beam time at ma CW with a single wafer, which was activated into a photocathode a total of 9 times in 36 months of operation with an average of 6 recesiations per activation GaAs wafer 25 mm dia Active area 16 mm dia Drive laser 8 mm dia The picture shows the photocathode being illuminated with the drive laser

4 The 1/e photocathode lifetime in the FEL gun is 50 hours or 550 Coulombs at an average current of 5 ma (and ~5x10-11 Torr) QE (% y = e x Extracted CW beam time (hours)

5 New GaAs-Based Photoinjector Initiatives with polarized electrons ILC and CLIC (demanding time structure and extremely high bunch charge) Electron Ion Collider Recently achieved (Jlab). Photocathode lifetime: about 7 days. ELIC (1mA within macropulse at 85% pol) erhic(25 to 250mA ave current at 85% pol) Photocathode lifetime: about 1 hour. Laser does not exists yet to achieve it.

6 GaAs-Based Photoinjector Initiatives with un-polarized electrons JLab IR FEL Daresbury ERLP JAEA ERL Cornell ERL JLab 100kW FEL (10mA and 350kV) (modest current, 350kV) (50mA and 250kV) (100mA and 750kV) (100mA and 500kV) ONLY about 1 hour of beam time with the present photocathode lifetime!!!

7 To face the challenge ahead for higher avg power and brighter electron beams The DC photogun community is working towards reducing the most pressing technological issue, high voltage (>500kV) stand-off

8 and oh! Yes, once that s solved, take care of: Improving photocathode lifetime for more run-time and happy users Better vacuum conditions Reduce halo at high average current Novel cathodes??? Improving stable operation at high average current for minimizing repair downtime Developing lasers with proper pulse train and high average power for both, polarized and unpolarized beam

9 Now, some basics: Photoemission from GaAs* Bare GaAs surface; Large work function. No electrons Alkalai (Cs) reduces work function. Some electrons. Cesium + Oxidant (O or NF3) Negative Electron Affinity. Many electrons E g > 0 E g 0 E g < 0 * Courtesy of Matt Poelker. CEBAF Polarized Source Group.

10 Polarized electron beam requirements* Bake/Vent Guns Superlattice GaAs: Layers of GaAs on GaAsP 100 nm chekc 14 pairs Fiber-based Laser No strain relaxation QE ~ 0.8% Pol ~ 780 nm These items exist at many locations, with happy Users worldwide.. * Courtesy of Matt Poelker. CEBAF Polarized Source Group.

11 In a DC photogun, electron bunches are generated when the GaAs photocathode is illuminated with pulses from a drive laser

12 The FEL is driven by a 350 kv DC GaAs electron gun, the highest average current photoemission source to ever drive an FEL RGA, extractor gauge and leak valve Vacuum chamber Electrodes Ceramic stand-offs Corona shield Photocathode retractor mechanism Photocathode NEG pumps High voltage feed 33 inches

13 We presently operate with bare, polished stainless steel electrodes. BIG PROBLEM in all DC guns is field emission which can puncture ceramics Support tube, 12.4 MV/m Ball cathode, ~ 8 MV/m GaAs wafer (6.0 MV/m) is activated into a NEA photocathode by depositing Cs from INSIDE the Ball cathode 25 cm Note: Field strengths calculated for 500 kv

14 A multi-national collaboration is underway to devise a robust electrode/insulator solution Cornell, JLab and Daresbury are manufacturing partially conductive ceramics to prevent charge accumulation which leads to punctures, but brazing large diameters is a problem. e-

15 The Japanese Atomic Energy Agency has tested segmented insulators up to 250kV. Their new DC gun will be tested up to 500kV next spring. Cornell just ordered a 750kV version with Kyocera to replace their cylindrical insulator.

16 A new approach to prevent field emitted electrons from puncturing the insulator is to replace the tube-electrode / cylindrical-insulator pair with an inverted, cone-shaped insulator Present Ceramic Exposed to field emission Large area Expensive (~$50k) New Ceramic Limited FE Compact ~$5k e - New design Present design Medical x-ray technology Courtesy of Matt Poelker, JLab CEBAF

17 JLab CEBAF s 250kV system is very compact and will have a load-lock chamber 4 in Conventional geometry: cathode electrode mounted on metal support structure Replace conventional ceramic insulator with Inverted insulator: no SF6 and no HV breakdown outside chamber Work of Ken Surles-Law, Jefferson Lab

18 Inverted gun status at JLab CEBAF is making field emission measurements using inverted insulator and 225kV supply. So far, maximum applied voltage 215kV. Routine operation > 130kV FEL team is doing preliminary electrostatic modeling for its new 500kV gun concept

19 Before we can focus on further electron beam characterization, we need to solve the reliability of DC guns operating at very high voltages CRITICAL TESTS: Ceramics, ceramics and more ceramics (bulk conductivity, braze large diameters, inverted and segmented insulators) Reduce electrode field emission (electrode surface prep) NEXT STAGE DEVELOPMENTS Everybody going with load-lock systems Improve vacuum conditions for better cathode lifetime Optimize electrode geometry for better beam quality

20 and yet, somehow, life goes on THANK YOU.